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| United States Patent |
5,596,592
|
|
Tanigami
, et al.
|
January 21, 1997
|
Semiconductor laser device
Abstract
A semiconductor laser device includes a ridge waveguide having two side
surfaces, a crystalline burying layer disposed at both side surfaces of
the ridge waveguide, and a second conductivity type contact layer disposed
on the burying layer and the ridge waveguide. The burying layer includes a
first conductivity type first current blocking layer in contact with the
side surfaces of the ridge waveguide, a second conductivity type second
current blocking layer disposed on a portion of the first current blocking
layer and separated from the ridge waveguide by a portion of the first
current blocking layer near the ridge waveguide, a first conductivity type
third current blocking layer disposed on a portion of the first current
blocking layer near the ridge waveguide and on the second current blocking
layer, and a second conductivity type final burying layer disposed on the
third current blocking layer. In this structure, there is no pn junction
at a regrowth interface between the final burying layer and the contact
layer so that reduction in the forward voltage of the pn junction in
continuous operation is avoided and increasing leakage current is
suppressed so that the threshold current and light output of the laser do
not deteriorate over time.
| Inventors:
|
Tanigami; Yoriko (Itami, JP);
Kadowaki; Tomoko (Itami, JP);
Takemoto; Akira (Itami, JP)
|
| Assignee:
|
Mitsubish Denki Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
515000 |
| Filed:
|
August 14, 1995 |
Foreign Application Priority Data
| Current U.S. Class: |
372/46.01 |
| Intern'l Class: |
H01S 003/19 |
| Field of Search: |
372/46,45,44
|
References Cited
U.S. Patent Documents
| 4637845 | Jan., 1987 | Hirano | 148/171.
|
| 5383213 | Jan., 1995 | Irikawa et al. | 372/46.
|
| 5390205 | Feb., 1995 | Mori et al. | 372/46.
|
| 5426658 | Jun., 1995 | Kaneno et al. | 372/46.
|
| 5452315 | Sep., 1995 | Kimura et al. | 372/46.
|
| Foreign Patent Documents |
| 59-117288 | Jul., 1984 | JP.
| |
| 62-159488 | Jul., 1987 | JP.
| |
| 1300581 | Dec., 1989 | JP.
| |
Primary Examiner: Davie; James W.
Attorney, Agent or Firm: Leydig, Voit & Mayer
Claims
What is claimed is:
1. A semiconductor laser device including:
a ridge waveguide having two side surfaces;
a burying crystalline layer disposed at both side surfaces of the ridge
waveguide; and
a contact layer disposed on the burying crystalline layer and the ridge
waveguide, the burying crystalline layer comprising a first current
blocking layer in contact with the side surfaces of the ridge waveguide
and comprising a first conductivity type semiconductor, a second current
blocking layer disposed on a portion of the first current blocking layer
and separated from the ridge waveguide by a portion of the first current
blocking layer proximate the ridge waveguide, and comprising a second
conductivity type semiconductor, opposite the first conductivity type, a
third current blocking layer disposed on the portion of the first current
blocking layer proximate the ridge waveguide and on the second current
blocking layer and comprising a first conductivity type semiconductor, and
a final burying layer disposed on the third current blocking layer and
comprising a second conductivity type semiconductor, the contact layer
having the second conductivity type semiconductor.
2. A semiconductor laser device including:
a ridge waveguide having two side surfaces;
a burying crystalline layer disposed both side surfaces of the ridge
waveguide; and
a contact layer disposed on the burying crystalline layer and the ridge
waveguide, the ridge waveguide comprising a first cladding layer
comprising a first conductivity type semiconductor, an active layer
disposed on the first cladding layer, and a second cladding layer disposed
on the active layer, a main portion of the cladding layer comprising a
semiconductor having a second conductivity type, opposite the first
conductivity type, the burying crystalline layer comprising a first
current blocking layer in contact with the side surfaces of the ridge
waveguide and comprising a first conductivity type semiconductor, a second
current blocking layer disposed on a portion of the first current blocking
layer and separated from the ridge waveguide by a portion of the first
current blocking layer proximate the ridge waveguide, and comprising a
second conductivity type semiconductor, and a third current blocking layer
disposed on the portion of the first current blocking layer proximate the
ridge waveguide and on the second current blocking layer and comprising a
first conductivity type semiconductor; wherein
a main portion of the contact layer has the second conductivity type;
a portion of the first current blocking layer in contact with the ridge
waveguide has a carrier concentration higher than that of the main portion
of the second cladding layer, and a thin portion of the second cladding
layer at an interface between the second cladding layer and the first
current blocking layer has the first conductivity type;
a portion of the third current blocking layer in contact with the contact
layer has a carrier concentration higher than that of a portion of the
contact layer, and a thin portion of the contact layer at an interface
between the contact layer and the third current blocking layer has the
first conductivity type; and
a portion of the third current blocking layer in contact with the second
current blocking layer has a carrier concentration lower than that of a
portion of the third current blocking layer in contact with the contact
layer.
3. A semiconductor laser device including:
a ridge waveguide having two side surfaces;
a burying crystalline layer disposed at both side surfaces of the ridge
waveguide; and
a contact layer disposed on the burying crystalline layer and the ridge
waveguide, the ridge waveguide comprising a first cladding layer
comprising a first conductivity type semiconductor, an active layer
disposed on the first cladding layer, and a second cladding layer disposed
on the active layer, a main portion of the cladding layer comprising a
semiconductor having a second conductivity type opposite the first
conductivity type, the burying crystalline layer comprising a first
current blocking layer in contact with the side surfaces of the ridge
waveguide and comprising a first conductivity type semiconductor, a second
current blocking layer disposed on a portion of the first current blocking
layer and separated from the ridge waveguide by a portion of the first
current blocking layer proximate the ridge waveguide, and comprising a
second conductivity type semiconductor, and a third current blocking layer
disposed on the portion of the first current blocking layer proximate the
ridge waveguide and on the second current blocking layer and comprising a
first conductivity type semiconductor; wherein
a main portion of the contact layer comprises a second conductivity type
semiconductor;
the first current blocking layer has a carrier concentration higher than
that of a main portion of the second cladding layer, and a thin portion of
the second cladding layer at an interface between the second cladding
layer and the first current blocking layer has the first conductivity
type;
the first conductivity type semiconductor of the third current blocking
layer has a carrier concentration higher than that of the main portion of
the contact layer, and a thin portion of the contact layer at an interface
between the contact layer and the third current blocking layer has the
first conductivity type; and
the second current blocking layer includes impurities that are electrically
neutral and interstitial.
4. The semiconductor laser device of claim 3 wherein the impurities that
are electrically neutral are included only in thin layer portions of the
second current blocking layer in contact with the first and third current
blocking layers.
5. A semiconductor laser device including:
a ridge waveguide having two side surfaces;
a burying crystalline layer disposed both side surfaces of the ridge
waveguide; and
a contact layer disposed on the burying crystalline layer and the ridge
waveguide, the ridge waveguide comprising a first cladding layer
comprising a p type semiconductor, an active layer disposed on the first
cladding layer, and a second cladding layer disposed on the active layer,
a main portion of the second cladding layer comprising an n type
semiconductor, the burying crystalline layer comprising a first current
blocking layer in contact with the side surfaces of the ridge waveguide
and comprising a p type semiconductor, a second current blocking layer
disposed on a portion of the first current blocking layer, separated from
the ridge waveguide by the portion of the first current blocking layer
proximate the ridge waveguide, and comprising an n type semiconductor, and
a third current blocking layer disposed on a portion of the first current
blocking layer proximate the ridge waveguide on the second current
blocking layer, and comprising a p type semiconductor; wherein
a main portion of the contact layer is n type;
the p type semiconductor of the first current blocking layer has a carrier
concentration higher than that of a main portion of the second cladding
layer, and a thin portion of the second cladding layer at an interface
between the second cladding layer and the first current blocking layer is
p type;
the p type semiconductor of the third current blocking layer has a carrier
concentration higher than that of the main portion of the contact layer,
and a thin portion of the contact layer at an interface between the
contact layer and the third current blocking layer is p type; and
the second current blocking layer includes impurities serving as hole
traps.
6. The semiconductor laser device of claim 5 wherein the impurities serving
as the hole traps are included only in thin layer portions of the second
current blocking layer in contact with the first and third current
blocking layers.
Description
FIELD OF THE INVENTION
The present invention relates to a semiconductor laser device and a method
of fabricating the semiconductor laser device.
BACKGROUND OF THE INVENTION
A conventional semiconductor laser device that includes a ridge waveguide,
current blocking layers disposed on both sides of the ridge waveguide, and
a contact layer, and a method of fabricating this semiconductor device are
described below.
FIGS. 11(a)-11(e) are sectional views illustrating process steps in a
method of fabricating the conventional semiconductor laser device.
Initially, a p type InP first cladding layer 1, an InGaAsP active layer 2,
and an n type InP second cladding layer 3 are successively epitaxially
grown on a p type InP substrate. Then, an insulating film 4 comprising SiO
is deposited on a center portion of the second cladding layer 3 and, using
the insulating film as a mask, the first cladding layer 1, the active
layer 2, and the second cladding layer 3 are selectively etched to form a
ridge waveguide (figure 11(a)). In the step of FIG. 11(b), a p type InP
first current blocking layer 12 having a high charge carrier concentration
is selectively epitaxially grown on both sides of the ridge waveguide.
Further, as shown in FIG. 11(c), an n type InP second current blocking
layer 6 is selectively epitaxially grown on the p type InP first current
blocking layer 12. In this growth step, the second current blocking layer
6 grows only on a specific crystalline plane of the first current blocking
layer 12. Therefore, it is possible to form the n type InP second current
blocking layer 6 so that it is not in contact with the side surface of the
ridge waveguide. Then, a p type InP third current blocking layer 13 having
a high charge carrier concentration is selectively epitaxially grown on
the first and second current blocking layers to form a crystalline burying
layer comprising the first, second, and third current blocking layers
(FIG. 11(d)). In the step of FIG. 11(e), the insulating film 4 is removed.
Finally, an n type InP contact layer 8 is epitaxially grown, followed by
grinding at the rear surface of the substrate, and formation of an
electrode 20a on the ground rear surface of the substrate and an electrode
20b on the n type InP contact layer 8, completing the semiconductor laser
device shown in FIG. 12.
In this semiconductor laser device, when a forward bias voltage is applied
across the electrodes 20a and 20b, a current flows through the ridge
waveguide comprising the n type InP second cladding layer 3, the active
layer 2, and the p type InP first cladding layer 1, and holes from the p
type InP first cladding layer 1 and electrons from the n type InP second
cladding layer 3 are injected into the active layer 2. Radiative
recombination of the electrons with the holes produces light in the active
layer 2, resulting in laser oscillation.
In the fabrication method described, since the layers of the ridge
waveguide and the burying layer are continuously epitaxially grown, no
surface of the grown layers is exposed to air during growth. However, the
p type InP first current blocking layer 12 is regrown on a side surface of
the ridge waveguide that is exposed to air during etching, and the n type
InP contact layer 8 is regrown on the upper surfaces of the burying layer
and the ridge waveguide that have been exposed to air during the etching
of the insulating film 4. The surfaces where regrowth occurs are called
regrowth interfaces.
It is known that when the p-n junction between the p type InP first current
blocking layer 12 and the n type InP second cladding layer 3 and the p-n
junction between the p type InP third current blocking layer 13 and the n
type InP contact layer 8 are located at regrowth interfaces, leakage
current not passing through the active layer increases. Thus, the forward
voltage at which a forward current starts to flow is reduced and the
forward current across the p-n junction under continuous operation
increases, causing deterioration of laser characteristics, such as a rise
in the threshold current and a reduction in light output. In order to
avoid this problem, Zn is employed as the dopant impurity in the p type
InP first and third current blocking layers 12 and 13. These p type
current blocking layers have significantly higher charge carrier
concentrations than the n type cladding layer 3 and the n type contact
layer 8. The Zn diffuses from the p type current blocking layers 12 and 13
into the n type cladding layer 3 and the n type contact layer 8 during the
epitaxial growth process or during heat treatment after the epitaxial
growth process, reversing the conductivity type of a thin portion of the n
type cladding layer 3 and the n type contact layer 8 in contact with
regrowth interfaces. Therefore, the p-n junction between the p type InP
third current blocking layer 13 and the n type InP contact layer 8 is not
located at the regrowth interface 9a of the upper portion of the burying
layer shown in FIG. 12, but at a position 14a in the n type contact layer
8. The p-n junction between the p type InP first current blocking layer 12
and the n type InP second cladding layer 3 is not located at the regrowth
interface 9b of the side surface of the n type cladding layer 3, but at a
position 14b in the n type cladding layer 3. Consequently, the forward
voltage of the p-n junction under continuous operation is not reduced and
does not cause deterioration of laser characteristics.
Although deterioration of the laser characteristics under continuous
operation can be prevented by diffusing Zn from the p type current
blocking layers 12 and 13 into the n type cladding layer 3 and the n type
contact layer 8, Zn diffuses not only into the n type cladding layer 3 and
the n type contact layer 8, but also into the n type InP second current
blocking layer 6. Zn compensates the dopant impurities of the n type
second current blocking layer 6, whereby the charge carrier concentration
of the n type current blocking layer 6 is reduced and the current blocking
effect due to the p-n-p transistor effect is reduced. That compensation
causes an increase in leakage current and deterioration of laser
characteristics.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of fabricating
a semiconductor laser device in which deterioration of laser
characteristics under continuous operation are prevented without reducing
the current blocking effect of the current blocking layers, and a
semiconductor laser device fabricated by the method.
Other objects and advantages of the present invention will become apparent
from the detailed description given hereinafter; it should be understood,
however, that the detailed description and specific embodiment are given
by way of illustration only, since various changes and modifications
within the scope of the invention will become apparent to the those
skilled in the art from this detailed description.
According to a first aspect of the present invention, a method of
fabricating a semiconductor laser device includes forming a ridge
waveguide comprising a first cladding layer, an active layer, and a second
cladding layer, successively growing by selective epitaxial growth a first
current blocking layer comprising a first conductivity type semiconductor,
a second current blocking layer separated from the ridge waveguide by a
portion of the first current blocking layer in the vicinity of the ridge
waveguide and comprising a second conductivity type semiconductor, a third
current blocking layer comprising a first conductivity type semiconductor,
and a final burying layer, comprising a second conductivity type
semiconductor, on both sides of the ridge waveguide to form a crystalline
burying layer, and epitaxially growing a contact layer comprising a second
conductivity type semiconductor on the second cladding layer and on the
final burying layer. Since a p-n junction is not formed at the regrowth
interface between the final burying layer and the contact layer, a
reduction in the forward voltage of the p-n junction under continuous
operation is avoided and an increase in leakage current is suppressed,
preventing deterioration of laser characteristics, such as a rise in a
threshold current and a reduction in light output. Since there is no
diffusion of dopant impurities from the third current blocking layer of
the first conductivity type into the contact layer, no dopant impurities
diffuse from the first and third current blocking layers into the second
current blocking layer of the second conductivity type, and no
deterioration of the current blocking effect of the current blocking
layers due to a reduction of the charge carrier concentration of the
second current blocking layer occurs.
According to a second aspect of the present invention, a method of
fabricating a semiconductor laser device includes successively epitaxially
growing a first cladding layer comprising a first conductivity type
semiconductor, an active layer, and a second cladding layer comprising a
second conductivity type semiconductor on a front surface of a
semiconductor substrate of the first conductivity type, depositing an
insulating film on a prescribed region of the second cladding layer,
selectively etching the first cladding layer, the active layer, and the
second cladding layer using the insulating film as a mask to form a ridge
waveguide, successively growing by selective epitaxial growth a lower
layer of a first current blocking layer comprising a first conductivity
type semiconductor having a charge carrier concentration higher than that
of the second conductivity type second cladding layer, an upper layer of a
first current blocking layer comprising a first conductivity type
semiconductor having a charge carrier concentration lower than that of the
lower layer of the first current blocking layer, a second current blocking
layer separated from the ridge waveguide by a portion of the first current
blocking layer in the vicinity of the ridge waveguide and comprising a
second conductivity type semiconductor, a lower layer of a third current
blocking layer comprising a first conductivity type semiconductor, and an
upper layer of a third current blocking layer, comprising a first
conductivity type semiconductor having a carrier concentration higher than
those of the lower layer of the third current blocking layer and the
second conductivity type contact layer, on both sides of the ridge
waveguide to form a crystalline burying layer, removing the insulating
film, and epitaxially growing a contact layer comprising a second
conductivity type semiconductor on the second cladding layer and on the
third current blocking layer. Due to the heating while forming the
crystalline burying layer by epitaxial growth and while epitaxially
growing the contact layer or in a heat treatment after the epitaxial
growth of the contact layer, dopant impurities diffuse from the first and
third current blocking layers to the second cladding layer and the contact
layer, respectively, whereby a thin portion of the second cladding layer
in contact with the first current blocking layer and a thin portion of the
contact layer in contact with the third current blocking layer are
reversed to the first conductivity type. Therefore, a p-n junction between
the contact layer and the third current blocking layer moves from the
regrowth interface between these layers to a position in the contact
layer, and a p-n junction between the second cladding layer and the first
current blocking layer moves from the regrowth interface between these
layers to a position in the second cladding layer. Consequently, in the
semiconductor laser device fabricated by this method, a reduction in a
forward voltage of the p-n junction under continuous operation is avoided,
preventing deterioration of laser characteristics, such as a rise in
threshold current and a reduction in light output. Meanwhile, since the
upper layer of the first current blocking layer and the lower layer of the
third current blocking layer in contact with the second current blocking
layer have a lower charge carrier concentration, the dopant impurities
diffused from the first and third current blocking layers into the second
current blocking layer are few, whereby deterioration of the current
blocking effect of the current blocking layers due to a reduction of the
charge carrier concentration of the second current blocking layer is
prevented.
According to a third aspect of the present invention, a fabricating method
of a semiconductor laser device includes successively epitaxially growing
a first cladding layer comprising a first conductivity type semiconductor,
an active layer, and a second cladding layer comprising a second
conductivity type semiconductor on a front surface of a semiconductor
substrate of the first conductivity type, depositing an insulating film on
a region of the second cladding layer, selectively etching the first
cladding layer, the active layer, and the second cladding layer using the
insulating film as a mask to form a ridge waveguide, successively growing
by selective epitaxial growth a first current blocking layer comprising a
first conductivity type semiconductor having a charge carrier
concentration higher than that of the second cladding layer, a second
current blocking layer separated from the ridge waveguide by a portion of
the first current blocking layer in the vicinity of the ridge waveguide
and comprising a second conductivity type semiconductor, and a third
current blocking layer comprising a first conductivity type semiconductor
having a charge carrier concentration higher than that of a second
conductivity type contact layer on both sides of the ridge waveguide to
form a crystalline burying layer, removing the insulating film, and
epitaxially growing a contact layer comprising the second conductivity
type semiconductor on the second cladding layer and on the third current
blocking layer. In the crystalline burying layer forming process, the
second current blocking layer is selectively epitaxially grown to include
impurities that are electrically neutral and interstitial in the crystal
lattice. Due to the heating while forming the crystalline burying layer by
selective epitaxial growth and while epitaxially growing the contact layer
or in a heat treatment after the epitaxial growth of the contact layer,
dopant impurities diffuse from the first and third current blocking layers
into the second cladding layer and the contact layer, respectively,
whereby a thin portion of the second cladding layer in contact with the
first current blocking layer and a thin portion of the contact layer in
contact with the third current blocking layer are reversed to the first
conductivity type. Accordingly, a p-n junction between the contact layer
and the third current blocking layer moves from the regrowth interface
between these layers to a position in the contact layer, and a p-n
junction between the second cladding layer and the first current blocking
layer moves from the regrowth interface between these layers to a position
in the second cladding layer. Therefore, in the semiconductor laser device
fabricated by this method, a reduction in a forward voltage of the p-n
junction under continuous operation is avoided, preventing deterioration
of laser characteristics, such as a rise in threshold current and a
reduction in light output. Meanwhile, since the second current blocking
layer includes impurities that are electrically neutral and interstitial
in the crystal lattice, the neutral impurities prevent the dopant impurity
from being interstitial in the crystal lattice. Consequently, the
diffusion of the dopant impurities from the first and third current
blocking layers into the second current blocking layer of the second
conductivity type is suppressed, whereby deterioration of the current
blocking effect of the current blocking layers due to a reduction of the
charge carrier concentration of the second current blocking layer is
prevented.
According to a fourth aspect of the present invention, in the described
method of fabricating the semiconductor laser device, the selective
epitaxial growth of the second current blocking layer in the crystalline
burying layer forming process comprises successively growing a lower layer
comprising a second conductivity type semiconductor including the
impurities that are electrically neutral and interstitial in the crystal
lattice, an intermediate layer comprising a semiconductor including only
the dopant impurities, and an upper layer comprising a second conductivity
type semiconductor including the impurities that are electrically neutral
and interstitial in the crystal lattice. Therefore, in the semiconductor
laser device fabricated by this method, deterioration of laser
characteristics under continuous operation is prevented by the diffusion
of the dopant impurities from the first and third current blocking layers
to the second cladding layer and the contact layer, respectively.
Meanwhile, since the upper and lower layers of the second current blocking
layer including the neutral impurities are in contact with the third and
first current blocking layers, respectively, the diffusion of the dopant
impurities from the first and third current blocking layers to the second
current blocking layer is suppressed, whereby a deterioration of the
current blocking effect of the current blocking layers due to a reduction
of the charge carrier concentration of the second current blocking layer
is prevented. Although the neutral impurities tend to obstruct the
activation of the dopant impurities in the second current blocking layer,
the intermediate layer of the second current blocking layer includes the
dopant impurities but it does not include the neutral impurities.
Consequently, the charge carrier concentration is higher than when the
neutral impurities are introduced into the entirety of the second current
blocking layer as described above, whereby deterioration of the current
blocking effect of the current blocking layers is further suppressed.
According to a fifth aspect of the present invention, a fabricating method
of a semiconductor laser device includes successively epitaxially growing
a first cladding layer comprising a p type semiconductor, an active layer,
and a second cladding layer comprising an n type semiconductor on a front
surface of a p type semiconductor substrate, depositing an insulating film
on a region of the second cladding layer, selectively etching the first
cladding layer, the active layer, and the second cladding layer using the
insulating film as a mask to form a ridge waveguide, successively growing
by selective epitaxial growth a first current blocking layer comprising a
p type semiconductor having a charge carrier concentration higher than
that of the second cladding layer, a second current blocking layer
separated from the ridge waveguide by a portion of the first current
blocking layer in the vicinity of the ridge waveguide and comprising an n
type semiconductor, and a third current blocking layer comprising a p type
semiconductor having a charge carrier concentration higher than that of a
contact layer on both sides of the ridge waveguide to form a crystalline
burying layer, removing the insulating film, and epitaxially growing a
contact layer comprising an n type semiconductor on the second cladding
layer and on the third current blocking layer. In the crystalline burying
layer forming process, the second current blocking layer is selectively
epitaxially grown to include impurities serving as hole traps. Due to the
heating while forming the crystalline burying layer by selective epitaxial
growth and while epitaxially growing the contact layer or in a the heat
treatment after the epitaxial growth of the contact layer, dopant
impurities diffuse from the first and third current blocking layers to the
second cladding layer and the contact layer, respectively, whereby a thin
portion of the second cladding layer in contact with the first current
blocking layer and a thin portion of the contact layer in contact with the
third current blocking layer are reversed to p type. Accordingly, a p-n
junction between the contact layer and the third current blocking layer
moves from the regrowth interface between these layers to a position in
the contact layer, and a p-n junction between the second cladding layer
and the first current blocking layer moves from the regrowth interface
between these layers to a position in the second cladding layer.
Therefore, in the semiconductor laser device fabricated by this method, a
reduction in the forward voltage of the p-n junction under continuous
operation is avoided, preventing deterioration of laser characteristics,
such as a rise in threshold current and a reduction in light output.
Meanwhile, since the second current blocking layer includes the impurities
serving as hole traps, the diffusion of the holes from the first and third
current blocking layers to the second current blocking layer of the n type
semiconductor is suppressed, whereby deterioration of the current blocking
effect of the current blocking layers is prevented.
According to a sixth aspect of the present invention, in the described
method of fabricating the semiconductor laser device, the selective
epitaxial growth of the second current blocking layer in the crystalline
burying layer forming process comprises successively growing a lower layer
comprising an n type semiconductor including the impurities serving as
hole traps, an intermediate layer comprising a semiconductor including
only dopant impurities, and an upper layer comprising an n type
semiconductor including the impurities serving as hole traps. Therefore,
in the semiconductor laser device fabricated by this method, as described
above, deterioration of laser characteristics under continuous operation
is prevented by the diffusion of dopant impurities from the first and
third current blocking layers to the second cladding layer and the contact
layer, respectively. Meanwhile, since the upper layer and the lower layer
of the second current blocking layer including the impurities serving as
the hole traps are in contact with the third and first current blocking
layers, respectively, the diffusion of holes from the first and third
current blocking layers into the second current blocking layer is
suppressed by the upper and lower layers, whereby deterioration of the
current blocking effect of the current blocking layers is prevented.
Although the impurities serving as hole traps tend to obstruct the
activation of the dopant impurities in the second current blocking layer,
the intermediate layer of the second current blocking layer includes the
dopant impurities but it does not include the impurities serving as hole
traps. Consequently, the charge carrier concentration is higher than when
the impurities serving as hole traps are introduced into the entirety of
the second current blocking layer as described above, whereby the
deterioration of the current blocking effect of the current blocking
layers is further suppressed.
According to a seventh aspect of the present invention, a semiconductor
laser device includes a ridge waveguide, a burying layer disposed on both
sides of the ridge waveguide, and a contact layer disposed on the burying
layer and the ridge waveguide. The burying layer comprises a first current
blocking layer in contact with the side surface of the ridge waveguide and
comprising a first conductivity type semiconductor, a second current
blocking layer disposed on a portion of the first current blocking layer
except in the vicinity of the ridge waveguide, separated from the ridge
waveguide by the portion of the first current blocking layer in the
vicinity of the ridge waveguide, and comprising a second conductivity type
semiconductor, a third current blocking layer disposed on the portion of
the first current blocking layer in the vicinity of the ridge waveguide
and on the entire surface of second current blocking layer and comprising
a first conductivity type semiconductor, and a final burying layer
disposed on the third current blocking layer and comprising a second
conductivity type semiconductor. The contact layer comprises a second
conductivity type semiconductor. Therefore, since a p-n junction is not
formed at the regrowth interface between the final burying layer and the
contact layer, a reduction in the forward voltage of the p-n junction
under continuous operation is avoided, preventing deterioration of laser
characteristics, such as a rise in threshold current and a reduction in
light output. Meanwhile, since the dopant impurities from the third
current blocking layer of the first conductivity type semiconductor do not
diffuse into the contact layer, the dopant impurities are not diffused
from the first and third current blocking layers to the second current
blocking layer of the second conductivity type, whereby there is no
deterioration of the current blocking effect of the current blocking
layers due to a reduction of the charge carrier concentration of the
second current blocking layer.
According to an eighth aspect of the present invention, a semiconductor
laser device includes a ridge waveguide, a burying layer disposed on both
sides of the ridge waveguide, and a contact layer disposed over the entire
surface of the burying layer and the ridge waveguide. The ridge waveguide
comprises a first cladding layer comprising a first conductivity type
semiconductor, an active layer disposed on the first cladding layer, and a
second cladding layer disposed on the active layer, the main portion of
the cladding layer comprising a second conductivity type semiconductor.
The burying layer comprises a first current blocking layer in contact with
the side surface of the ridge waveguide and comprising a first
conductivity type semiconductor, a second current blocking layer disposed
on a portion of the first current blocking layer, except in the vicinity
of the ridge waveguide, separated from the ridge waveguide by the portion
of the first current blocking layer in the vicinity of the ridge
waveguide, and comprising a second conductivity type semiconductor, and a
third current blocking layer disposed on the portion of the first current
blocking layer in the vicinity of the ridge waveguide and on the entire
surface of second current blocking layer and comprising the first
conductivity type semiconductor. The main portion of the contact layer
comprises a second conductivity type semiconductor. The first conductivity
type semiconductor in the portion of the first current blocking layer in
contact with the ridge waveguide has a charge carrier concentration higher
than that of the second conductivity type semiconductor of the second
cladding layer, and a thin portion of the second cladding layer in the
vicinity of the interface between the second cladding layer and the first
current blocking layer comprises the same first conductivity type
semiconductor as the first current blocking layer. The first conductivity
type semiconductor in the portion of the first current blocking layer in
contact with the second current blocking layer has a charge carrier
concentration lower than that of the first conductivity type semiconductor
in the portion of the first current blocking layer in contact with the
ridge waveguide. The first conductivity type semiconductor in the portion
of the third current blocking layer in contact with the contact layer has
a charge carrier concentration higher than that of the second conductivity
type semiconductor in the main portion of the contact layer, and a thin
portion of the contact layer in the vicinity of the interface between the
contact layer and the third current blocking layer comprises the same
first conductivity type semiconductor as the third current blocking layer.
The first conductivity type semiconductor in the portion of the third
current blocking layer in contact with the second current blocking layer
has a charge carrier concentration lower than that of the first
conductivity type semiconductor in the portion of the third current
blocking layer in contact with the contact layer. Therefore, a p-n
junction between the contact layer and the third current blocking layer is
positioned in the contact layer, removed from the regrowth interface
between these layers, and a p-n junction between the second cladding layer
and the first current blocking layer is positioned in the second cladding
layer, removed from the regrowth interface between these layers.
Consequently, a reduction in the forward voltage of the p-n junction under
continuous operation is avoided, preventing deterioration of laser
characteristics, such as a rise in threshold current and a reduction in
light output. Meanwhile, since the first conductivity type semiconductor
in the portions of the first and third current blocking layers in contact
with the second current blocking layer has a lower charge carrier
concentration, dopant impurities diffusing from the first and third
current blocking layers to the second current blocking layer are few,
whereby deterioration of the current blocking effect of the current
blocking layers due to a reduction in charge carrier concentration in the
second current blocking layer is prevented.
According to a ninth aspect of the present invention, a semiconductor laser
device includes a ridge waveguide, a burying layer disposed on both sides
of the ridge waveguide, and a contact layer disposed on the burying layer
and the ridge waveguide. The ridge waveguide comprises a first cladding
layer comprising a first conductivity type semiconductor, an active layer
disposed on the first cladding layer, and a second cladding layer disposed
on the active layer, the main portion of the cladding layer comprising a
second conductivity type semiconductor. The burying layer comprises a
first current blocking layer in contact with the side surface of the ridge
waveguide and comprising a first conductivity type semiconductor, a second
current blocking layer disposed on a portion of the first current blocking
layer except in the vicinity of the ridge waveguide, separated from the
ridge waveguide by the portion of the first current blocking layer in the
vicinity of the ridge waveguide, and comprising a second conductivity type
semiconductor, and a third current blocking layer disposed on the portion
of the first current blocking layer in the vicinity of the ridge waveguide
and on the second current blocking layer and comprising a first
conductivity type semiconductor. The main portion of the contact layer
comprises the second conductivity type semiconductor. The first
conductivity type semiconductor of the first current blocking layer has a
charge carrier concentration higher than that of the second conductivity
type semiconductor in the main portion of the second cladding layer, and a
thin portion of the second cladding layer in the vicinity of the interface
between the second cladding layer and the first current blocking layer
comprises the same first conductivity type semiconductor as the first
current blocking layer. The first conductivity type semiconductor of the
third current blocking layer has a charge carrier concentration higher
than that of the second conductivity type semiconductor in the main
portion of the contact layer, and a thin portion of the contact layer in
the vicinity of the interface between the contact layer and the third
current blocking layer comprises the same first conductivity type
semiconductor as the third current blocking layer. The second current
blocking layer includes impurities that are electrically neutral and
interstitial in the crystal lattice. Accordingly, a p-n junction between
the contact layer and the third current blocking layer is positioned in
the contact layer, removed from the regrowth interface between these
layers, and a p-n junction between the second cladding layer and the first
current blocking layer is positioned in the second cladding layer, removed
from the regrowth interface between these layers. Therefore, a reduction
in the forward voltage of the p-n junction under continuous operation is
avoided, preventing deterioration of laser characteristics, such as a rise
in threshold current and a reduction in light output. Meanwhile, since the
second current blocking layer includes the impurities that are
electrically neutral and interstitial in the crystal lattice, the neutral
impurities prevent the dopant impurities from diffusing into the crystal
lattice. Consequently, the diffusion of the dopant impurities from the
first and third current blocking layers to the second current blocking
layer of the second conductivity type is suppressed, whereby deterioration
of the current blocking effect of the current blocking layers due to a
reduction of the charge carrier concentration of the second current
blocking layer is prevented.
According to a tenth aspect of the present invention, in the semiconductor
laser device, the impurities that are electrically neutral are included
only in thin layer portions of the second current blocking layer in
contact with the first and third current blocking layers. Therefore, since
a thin portion of the second cladding layer and the contact layer in
contact with the first and third current blocking layers, respectively, is
of the first conductivity type, deterioration of laser characteristics
under continuous operation is prevented as described above. Meanwhile,
since the thin layer portions of the second current blocking layer
including the neutral impurities are in contact with the first and third
current blocking layers, the diffusion of the dopant impurities from the
first and third current blocking layers to the second current blocking
layer is suppressed, whereby deterioration of the current blocking effect
of the current blocking layers due to a reduction of charge carrier
concentration in the second current blocking layer is prevented. Although
the neutral impurities tend to obstruct the activation of the dopant
impurities in the second current blocking layer, a portion of the second
current blocking layer, except the thin layer portions, includes the
dopant impurities but does not include the neutral impurities.
Consequently, the charge carrier concentration is higher than when the
neutral impurities are introduced into the entirety of the second current
blocking layer as described above, whereby the deterioration of the
current blocking effect of the current blocking layers is further
suppressed.
According to an eleventh aspect of the present invention, a semiconductor
laser device includes a ridge waveguide, a crystalline burying layer
disposed on both sides of the ridge waveguide, and a contact layer
disposed on the crystalline burying layer and the ridge waveguide. The
ridge waveguide comprises a first cladding layer comprising a p type
semiconductor, an active layer disposed on the first cladding layer, and a
second cladding layer disposed on the active layer, the main portion of
the cladding layer comprising an n type semiconductor. The crystalline
burying layer comprises a first current blocking layer in contact with the
side surfaces of the ridge waveguide and comprising a p type
semiconductor, a second current blocking layer disposed on a portion of
the first current blocking layer, except in the vicinity of the ridge
waveguide, separated from the ridge waveguide by the portion of the first
current blocking layer in the vicinity of the ridge waveguide, and
comprising an n type semiconductor, and a third current blocking layer
disposed on the portion of the first current blocking layer in the
vicinity of the ridge waveguide and on the second current blocking layer
and comprising a p type semiconductor. The main portion of the contact
layer comprises an n type semiconductor. The p type semiconductor of the
first current blocking layer has a charge carrier concentration higher
than that of the n type semiconductor in the main portion of the second
cladding layer, and a thin portion of the second cladding layer in the
vicinity of the interface between the second cladding layer and the first
current blocking layer comprises the same p type semiconductor as the
first current blocking layer. The p type semiconductor of the third
current blocking layer has a charge carrier concentration higher than that
of the n type semiconductor in the main portion of the contact layer, and
a thin portion of the contact layer in the vicinity of the interface
between the contact layer and the third current blocking layer comprises
the same p type semiconductor as the third current blocking layer. The
second current blocking layer includes impurities serving as hole traps.
Therefore, a p-n junction between the contact layer and the third current
blocking layer is positioned in the contact layer, removed from the
regrowth interface between these layers, and a p-n junction between the
second cladding layer and the first current blocking layer is positioned
in the second cladding layer, removed from the regrowth interface between
these layers. Consequently, a reduction in the forward voltage of the p-n
junction under continuous operation is avoided, preventing deterioration
of laser characteristics, such as a rise in threshold current and a
reduction in light output. Meanwhile, since the second current blocking
layer includes the impurities serving as hole traps, the diffusion of
holes from the first and third current blocking layers to the second n
type current blocking layer is suppressed, whereby deterioration of the
current blocking effect of the current blocking layers is prevented.
According to a twelfth aspect of the present invention, in the
semiconductor laser device, the impurities serving as the hole traps are
included only in thin layer portions of the second current blocking layer
in contact with the first and third current blocking layers. Therefore,
since a thin portion of the second cladding layer and the contact layer in
contact with the first and third current blocking layers, respectively,
has the first conductivity type, deterioration of laser characteristics
under continuous operation is prevented as described above. Meanwhile,
since the thin layer portions of the second current blocking layer
including the impurities serving as hole traps are in contact with the
first and third current blocking layers, the diffusion of holes from the
first and third current blocking layers to the second current blocking
layer is suppressed, whereby deterioration of the current blocking effect
of the current blocking layers is prevented. Although the impurities
serving as hole traps tend to obstruct the activation of dopant impurities
in the second current blocking layer, a portion of the second current
blocking layer includes the dopant impurities but does not include the
impurities serving as hole traps. Consequently, the charge carrier
concentration is higher than when the impurities serving as hole traps are
introduced into the entirety of the second current blocking layer as
described above, whereby the deterioration of the current blocking effect
of the current blocking layers is further suppressed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 (a)-1 (f) are sectional views illustrating process steps in a
method of fabricating a semiconductor laser device in accordance with a
first embodiment of the present invention.
FIG. 2 is a sectional view illustrating a semiconductor laser device in
accordance with the first embodiment of the invention.
FIGS. 3 (a)-3 (e) are sectional views illustrating process steps in a
method of fabricating a semiconductor laser device in accordance with a
second embodiment of the present invention.
FIG. 4 is a sectional view illustrating a semiconductor laser device in
accordance with the second embodiment of the invention.
FIGS. 5 (a)-5 (e) are sectional views illustrating process steps in a
method of fabricating a semiconductor laser device in accordance with a
third embodiment of the present invention.
FIG. 6 is a sectional view illustrating a semiconductor laser device in
accordance with the third embodiment of the invention.
FIGS. 7 (a)-7 (c ) are sectional views illustrating process steps in a
method of fabricating a semiconductor laser device in accordance with a
fourth embodiment of the present invention.
FIG. 8 is a sectional view illustrating a semiconductor laser device in
accordance with the fourth embodiment of the invention.
FIG. 9 is a sectional view illustrating a semiconductor laser device in
accordance with a fifth embodiment of the present invention.
FIG. 10 is a sectional view illustrating a semiconductor laser device in
accordance with a sixth embodiment of the present invention.
FIGS. 11 (a)-11 (e) are sectional views illustrating process steps in a
method of fabricating a semiconductor laser device according to the prior
art.
FIG. 12 is a sectional view illustrating a semiconductor laser device
according to the prior art.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
FIGS. 1 (a)-1 (f) are sectional views illustrating process steps in a
method of fabricating a semiconductor laser device according to a first
embodiment of the invention. Initially, a p type InP first cladding layer
1 having a thickness of about 1 .mu.m, an undoped InGaAsP active layer 2
having a thickness of about 0.1 .mu.m, and an n type InP second cladding
layer 3 having a thickness of about 1 .mu.m are successively epitaxially
grown on a p type InP substrate 1, preferably by metal organic chemical
vapor deposition (hereinafter referred to as MOCVD). Thereafter, a
selective growth mask 4 comprising an SiO film and having a thickness of
about 0.1 .mu.m is formed on a portion of the n type InP second cladding
layer 3 and, using the selective growth mask 4, the first cladding layer
1, the undoped InGaAsP active layer 2, and the second cladding layer 3 are
selectively etched to form a ridge waveguide (FIG. 1 (a)).
A p type InP first current blocking layer 5 having a thickness of about 1
.mu.m (FIG. 1 (b)), an n type InP second current blocking layer 6 having a
thickness of about 1 .mu.m FIG. 1 (c )), a p type InP third current
blocking layer 7 having a thickness of about 1 .mu.m FIG. 1 (d)), and an n
type InP final burying layer 10 having a thickness of about 0.2 .mu.m FIG.
1 (e)) are successively grown by selective epitaxial growth, preferably
using MOCVD, to form a crystalline burying layer. In the step of FIG. 1
(f), after removal of the selective growth mask 4, an n type InP contact
layer 8 having a thickness of about 2 .mu.m is epitaxially grown on the
entire surface of the final burying layer 10. In this growth step, the n
type InP contact layer 8 has the same charge carrier concentration and
composition as the n type InP final burying layer 10. Finally, the rear
surface of the substrate is ground, and an electrode 20b comprising Cr/Au
and having a thickness of about 0.2 .mu.m and an electrode 20a comprising
AuZn/Au and having a thickness of about 0.2 .mu.m are formed on the n type
InP contact layer 8 and on the ground rear surface of the substrate,
respectively, completing the semiconductor laser device shown in FIG. 2.
The first and second cladding layers 1 and 3 and the first, second, and
third current blocking layers 5, 6, and 7 have a charge carrier
concentration of about 10.sup.18 cm.sup.-3, and the n type InP contact
layer 8 has a charge carrier concentration exceeding 10.sup.18 cm.sup.-3.
Preferably, Zn is employed as the dopant impurity producing p type
conductivity, and Si or S is employed as the dopant impurity producing n
type conductivity.
In a semiconductor laser device fabricated using the method of the first
embodiment, as shown in FIG. 2, a p-n junction 11 of the upper portion of
the current blocking layers is located at the interface between the p type
InP third current blocking layer 7 and the n type InP final burying layer
10. This interface is not the regrowth interface. 0n the other hand, a
junction between the n type layers is located at the interface 9a between
the n type InP final burying layer 10 and the n type InP contact layer 8,
i.e., the regrowth interface of the upper portion of the burying layer.
Therefore, in the p-n junction between the p type third current blocking
layer 7 and the n type final burying layer 10, the forward voltage is not
reduced under continuous operation, so there is no deterioration of the
laser characteristics.
In the first embodiment of the invention, since the n type InP final
burying layer 10 is employed, a p-n junction is not located at the
regrowth interface 9a of the upper portion of the burying crystal, and the
diffusion of dopant impurities as in the conventional fabricating method
does not occur. Therefore, a reduction in the charge carrier concentration
of the n-type InP second current blocking layer 6 and a reduction in the
current blocking effect of the current blocking layers due to the
diffusion of dopant impurities producing p type conductivity into the n
type second current blocking layer 6 do not occur. In this first
embodiment, the substrate is heated during the epitaxial growth steps to a
temperature at which the diffusion of the dopant impurities producing p
type conductivity does not occur, or the p type first and third current
blocking layers 5 and 7 have charge carrier concentrations in which the
conductivity type of the n type layers surrounding the p type first and
third current blocking layers is not reversed to p type.
Meanwhile, since the interface 9b between the n type InP second cladding
layer 3 and the p type InP first current blocking layer 5 is a portion of
the side surface of the ridge waveguide and is the regrowth interface of
the side surface of the second cladding layer 3 and the p-n junction is
located there, it is possible to reduce the forward voltage under
continuous operation.
In addition, in this first embodiment of the invention, although an InP
semiconductor laser device is described, the same fabricating method and
the same structure may be adopted in a semiconductor laser device
comprising another material, such as GaAs. The number of current blocking
layers need not be three
Embodiment 2
FIGS. 3 (a)-3 (e) are sectional views illustrating process seeps in a
method of fabricating a semiconductor laser device according to a second
embodiment of the invention. Initially, as shown in FIG. 3 (a), a ridge
waveguide comprising a p type InP first cladding layer 1, an undoped
InGaAsP active layer 2, and an n type InP second cladding layer 3 is
formed by the same method as in the first embodiment of the invention.
Then, a lower layer 5a of a p type InP first current blocking layer having
a high charge carrier concentration and a thin upper layer 5b of a p type
InP first current blocking layer having a low charge carrier concentration
are successively grown by selective epitaxial growth (FIG. 3 (b)).
Further, an n type InP second current blocking layer 6 is selectively
epitaxially grown (FIG. 3 (c )).
In the step of FIG. 3 (d), a thin lower layer 7b of a p type InP third
current blocking layer having a low charge carrier concentration and an
upper layer 7a of a p type InP third current blocking layer having a high
charge carrier concentration are successively grown by selective epitaxial
growth to form a crystalline burying layer comprising the first, second,
and third current blocking layers. The growth of these current blocking
layers is continuous, preferably using MOCVD. Then, as shown in FIG. 3
(e), the selective growth mask 4 is removed. Finally, as in the first
embodiment of the invention, an n type InP contact layer 8 is epitaxially
grown on the entire surface of the upper layer 7a, the rear surface of the
substrate 1' is ground, and an electrode 20b and an electrode 20a are
formed on the n type InP contact layer 8 and on the ground rear surface of
the substrate, respectively, completing the semiconductor laser device
shown in FIG. 4.
In this case, the lower layer 5a of the p type InP first current blocking
layer has a higher charge carrier concentration than the n type InP second
cladding layer 3, and the upper layer 7a of the p type InP third current
blocking layer has a higher charge carrier concentration than the n type
InP contact layer 8. Therefore, when the substrate is heated during the
epitaxial growth steps to a suitable temperature, dopant impurities
producing p type conductivity diffuse from the lower layer 5a of the first
current blocking layer and the upper layer 7a of the third current
blocking layer into the second cladding layer 3 and the contact layer 8.
The conductivity type of a thin portion of the second cladding layer 3 and
the contact layer 8 comprising n type InP in contact with the current
blocking layers are reversed to p type. In addition, this diffusion may be
produced by heat treatment after the epitaxial growth.
In the second embodiment of the invention, as shown in FIG. 4, due to the
diffusion of the dopant impurities producing p type conductivity, a p-n
junction between the upper layer 7a of the third current blocking layer
and the contact layer 8, i.e., a p-n junction with the upper portion of
the burying layer, is located at a position 14a in the contact layer 8,
removed from the interface 9a between the upper layer 7a of the current
blocking layer and the contact layer 8, i.e., the regrowth interface of
the upper portion of the burying layer. Likewise, a p-n junction between
the lower layer 5a of the first current blocking layer and the second
cladding layer 3, i.e., a p-n junction with the side surface of the second
cladding layer 3, is located at a position 14b in the second cladding
layer 3, removed from the interface 9b between the lower layer 5a of the
current blocking layer and the second cladding layer 3, i.e., the regrowth
interface on the side surface of the second cladding layer 3. Therefore,
as described above, in these p-n junctions, forward voltage is not reduced
under continuous operation, so there is no deterioration of the laser
characteristics.
Meanwhile, since the upper layer 5b of the first current blocking layer and
the lower layer 7b of the third current blocking layer comprising p type
InP in contact with the n type InP second current blocking layer 6 have
lower charge carrier concentrations than the lower layer 5a of the first
current blocking layer and the upper layer 7a of the third current
blocking layer, respectively, the dopant impurities producing p type
conductivity that diffuse into the second current blocking layer 6 are
few. Therefore, a reduction in charge carrier concentration of the n type
InP second current blocking layer 6 due to compensation by the dopant
impurities producing p type conductivity is suppressed, whereby the
current blocking effect of the current blocking layers is not
deteriorated.
In the second embodiment of the invention, the deterioration of laser
characteristics under continuous operation is prevented without reducing
the current blocking effect of the current blocking layers. In addition,
although an InP semiconductor laser device is described, the same
fabricating method and the same structure may be adopted in a
semiconductor laser device comprising another material, such as GaAs. The
number of current blocking layers need not be three.
Embodiment 3
FIGS. 5 (a)-5 (e) are sectional views illustrating process steps in a
method of fabricating a semiconductor laser device according to a third
embodiment of the invention. Initially, as shown in FIG. 5 (a), a ridge
waveguide comprising a p type InP first cladding layer 1, an undoped
InGaAsP active layer 2, and an n type InP second cladding layer 3 is
formed by the same method as in the first embodiment of the invention.
Then, a p type InP first current blocking layer 12 having a high charge
carrier concentration is selectively epitaxially grown (FIG. 5 (b)). In
the step of FIG. 5 (c ), an n type InP second current blocking layer 15
including dopant impurities producing n type conductivity and impurities
that are electrically neutral and interstitial in the crystal lattice,
such as Co and Ti, is selectively epitaxially grown. Further, a p type InP
third current blocking layer 13 having a high charge carrier concentration
is selectively epitaxially grown to form a crystalline burying layer
comprising the first, second, and third current blocking layers (FIG. 5
(d)). The growth of these current blocking layers is performed
continuously, preferably using MOCVD. Then, as shown in FIG. 5 (e), the
selective growth mask 4 is removed.
Finally, similar to the first embodiment of the invention, an n type InP
contact layer 8 is epitaxially grown over the entire surface of the
current blocking layer 13, the rear surface of the substrate is ground,
and an electrode 20b and an electrode 20a are formed on the n type InP
contact layer 8 and on the ground rear surface of the substrate,
respectively, completing the semiconductor laser device shown in FIG. 6.
In this case, the p type InP first current blocking layer 12 has a higher
charge carrier concentration than the n type InP second cladding layer 3,
and the p type InP third current blocking layer 13 has a higher charge
carrier concentration than the n type InP contact layer 8. Therefore, when
the substrate is heated during epitaxial growth to a suitable temperature,
dopant impurities producing p type conductivity diffuse from the first and
third current blocking layers 12 and 13 into the second cladding layer 3
and the contact layer 8. The conductivity type of a thin portion of the
second cladding layer 3 and the contact layer 8, comprising n type InP, in
contact with the current blocking layers is reversed to p type. In
addition, this diffusion may be carried out by heat treatment after the
epitaxial growth.
Also, as shown in FIG. 6, due to the diffusion of the dopant impurities
producing p type conductivity, a p-n junction between the third current
blocking layer 13 and the contact layer 8, i.e., a p-n junction with the
upper portion of the crystalline burying layer, is formed at a position
14a in the contact layer 8, removed from the interface 9a between the
third current blocking layer 13 and the contact layer 8, i.e., the
regrowth interface of the upper portion of the burying crystal. Likewise,
a p-n junction between the first current blocking layer 12 and the second
cladding layer 3, i.e., a p-n junction with the side surface of the second
cladding layer 3, is formed at a position 14b in the second cladding layer
3, removed from the interface 9b between the first current blocking layer
12 and the second cladding layer 3, i.e., the regrowth interface of the
side surface of the second cladding layer 3. Therefore, the forward
voltage of these p-n junctions is not reduced under continuous operation
and does not cause the deterioration of laser characteristics.
Meanwhile, although the dopant impurities diffused from the p type InP
first and third current blocking layers 12 and 13 into the n type InP
second current blocking layer 15 are interstitial in the crystal lattice,
since the second current blocking layer 15 includes impurities that are
electrically neutral and interstitial in the crystal lattice, the
diffusion of the dopant impurities is suppressed. Therefore, a reduction
in the charge carrier concentration of the second current blocking layer
15 due to compensation by the dopant impurities producing p type
conductivity of the dopant impurities producing n type conductivity in the
second current blocking layer 15 is significantly suppressed, and the
current blocking effect of the current blocking layers is not
deteriorated.
In the third embodiment of the invention, the deterioration of the laser
characteristics under continuous operation is prevented without reducing
the current blocking effect of the current blocking layers. In addition,
although an InP semiconductor laser device is described, the same
fabricating method and the same structure may be adopted in a
semiconductor laser device comprising another material, such as GaAs. The
number of current blocking layers need not be three.
Embodiment 4
FIGS. 7 (a)-7 (c ) are sectional views illustrating process steps In a
method of fabricating a semiconductor laser device according to a fourth
embodiment of the invention. Initially, as in the third embodiment of the
invention, after the formation of a ridge waveguide comprising a p type
InP first cladding layer 1, an undoped InGaAsP active layer 2, and an n
type InP second cladding layer 3, a p type InP first current blocking
layer 12 having a high charge carrier concentration is selectively
epitaxially grown. Then, a thin lower layer 15b of an n type InP second
current blocking layer including dopant impurities producing n type
conductivity and impurities that are electrically neutral and interstitial
in the crystal lattice, such as Co and Ti, is selectively epitaxially
grown (FIG. 7 (a)). Further, an intermediate layer 15a of an n type InP
second current blocking layer including dopant impurities producing n type
conductivity is selectively epitaxially grown (FIG. 7 (b)). In the step of
FIG. 7 (c ), a thin upper layer 15c of an n type InP second current
blocking layer including dopant impurities producing n type conductivity
and impurities that are electrically neutral and interstitial in the
crystal lattice, such as Co and Ti, is selectively epitaxially grown.
The following processes are the same as those in the third embodiment of
the invention. A p type InP third current blocking layer 13 having a high
charge carrier concentration is selectively epitaxially grown to form a
crystalline burying layer comprising the first, second, and third current
blocking layers. After removal of the selective growth mask 4, an n type
InP contact layer 8 is epitaxially grown on the current blocking layer 13,
the rear surface of the substrate is ground, and an electrode 20b and an
electrode 20a are formed on the n type InP contact layer 8 and on the
ground rear surface of the substrate, respectively, completing the
semiconductor laser device shown in FIG. 8. In addition, the growth of the
current blocking layers is performed continuously, preferably using MOCVD.
In this case, the p type InP first current blocking layer 12 has a higher
charge carrier concentration than the n type InP second cladding layer 3,
and the p type InP third current blocking layer 13 has a higher charge
carrier concentration than the n type InP contact layer 8. Therefore, when
the substrate is heated during epitaxial growth, dopant impurities
producing p type conductivity diffuse from the first and third current
blocking layers 12 and 13 to the second cladding layer 3 and the contact
layer 8, and the conductivity type of a thin portion of the second
cladding layer 3 and the contact layer 8, comprising n type InP, in
contact with the current blocking layers is reversed to p type. In
addition, this diffusion may be carried out by heat treatment after the
epitaxial growth.
In the fourth embodiment of the invention, as shown in FIG. 8, due to the
diffusion of the dopant Impurities producing p type conductivity, a p-n
junction between the third current blocking layer 13 and the contact layer
8, i.e., a p-n junction with the upper portion of the crystalline burying
layer, is formed at a position 14a in the contact layer 8, removed from
the interface 9a between the third current blocking layer 13 and the
contact layer 8, i.e., the regrowth interface of the upper portion of the
crystalline burying layer. Likewise, a p-n junction between the first
current blocking layer 12 and the second cladding layer 3, i.e., a p-n
junction with the side surface of the second cladding layer 3, is formed
at a position 14b in the second cladding layer 3, removed from the
interface 9b between the first current blocking layer 12 and the second
cladding layer 3, i.e., the regrowth interface of the side surface of the
second cladding layer 3. Therefore, as described above, in these p-n
junctions, the forward voltage is not reduced under continuous operation,
avoiding deterioration of laser characteristics.
Meanwhile, although the dopant impurities producing p type conductivity
that diffuse from the p type InP first and third current blocking layers
12 and 13 into the n type InP second current blocking layer 15 are
interstitial in the crystalline lattice, since the upper layer 15c and the
lower layer 15b of the second current blocking layer in contact with the
third and the first current blocking layers 13 and 12, respectively,
include impurities that are electrically neutral and interstitial in the
crystalline lattice, the diffusion of the dopant impurities is suppressed.
Therefore, a reduction in the charge carrier concentration of the second
current blocking layer 15 due to compensation by the dopant impurities is
significantly suppressed. Although the neutral impurities tend to obstruct
the activation of the dopant impurities in the second current blocking
layer, the intermediate layer 15a of the second current blocking layer
includes dopant impurities producing n type conductivity but it does not
include the neutral impurities. Consequently, the charge carrier
concentration is higher than when the neutral impurities are introduced
into the entirety of the second current blocking layer 15 as in the third
embodiment of the invention, whereby deterioration of the current blocking
effect of the current blocking layers is further suppressed.
In the fourth embodiment of the invention, deterioration of laser
characteristics under continuous operation is prevented without reducing
the current blocking effect of the current blocking layers.
In addition, although an InP semiconductor laser device is described, the
same fabricating method and the same structure may be adopted in a
semiconductor laser device comprising another material, such as GaAs. The
number of current blocking layers need not be three.
Embodiment 5
FIG. 9 is a sectional view illustrating a semiconductor laser device
according to a fifth embodiment of the invention. In the figure, reference
numeral 16 designates an n type InP second current blocking layer
including dopant impurities and impurities serving as hole traps. In this
semiconductor laser device, the impurities serving as hole traps
substitute for the impurities that are electrically neutral and
interstitial in the crystalline lattice that are included in the n type
InP second current blocking layer 16 in the third embodiment of the
invention. A fabricating method according to the fifth embodiment of the
invention is the same as in the third embodiment, except that InP is grown
to include dopant impurities producing n type conductivity and impurities
serving as hole traps, such as Co, during the epitaxial growth of the
second current blocking layer 16. In this case, the p type InP first
current blocking layer 12 has a higher charge carrier concentration than
the n type InP second cladding layer 3, and the p type InP third current
blocking layer 13 has a higher charge carrier concentration than the n
type InP contact layer 8. Therefore, when the substrate is heated during
epitaxial growth to a suitable temperature, dopant impurities producing p
type conductivity diffuse from the first and third current blocking layers
12 and 13 into the second cladding layer 3 and the contact layer 8, and
the conductivity type of a thin portion of the second cladding layer 3 and
the contact layer 8 comprising n type InP in contact with the current
blocking layers is reversed to p type. In addition, this diffusion may be
carried out by heat treatment after the epitaxial growth.
In the fifth embodiment of the invention, as shown in FIG. 9, due to the
diffusion of the dopant impurities, a p-n junction between the third
current blocking layer 13 and the contact layer 8, i.e., a p-n junction
with the upper portion of the burying layer, is formed at a position 14a
in the contact layer 8 removed from the interface 9a between the third
current blocking layer 13 and the contact layer 8, i.e., the regrowth
interface of the upper portion of the burying layer. Likewise, a p-n
junction between the first current blocking layer 12 and the second
cladding layer 3, i.e., a p-n junction with the side surface of the second
cladding layer 3, is formed at a position 14b in the second cladding layer
3 removed from the interface 9b between the first current blocking layer
12 and the second cladding layer 3, i.e., the regrowth interface of the
side surface of the second cladding layer 3. Therefore, as described
above, in these p-n junctions, the forward voltage is not reduced under
continuous operation, so there is no deterioration of laser
characteristics.
Meanwhile, since the second current blocking layer 16 includes the
impurities serving as hole traps, the diffusion of the holes from the p
type InP first and third current blocking layers 12 and 13 to the n type
InP second current blocking layer 16 is suppressed, whereby the current
blocking effect of the current blocking layers is not deteriorated.
In the fifth embodiment of the invention, the deterioration of the laser
characteristics under continuous operation is prevented without reducing
the current blocking effect of the current blocking layers.
In addition, although an InP semiconductor laser device is described, the
same fabricating method and the same structure may be adopted in a
semiconductor laser device comprising another material, such as GaAs. The
number of current blocking layers need not be three.
Embodiment 6
FIG. 10 is a sectional view illustrating a semiconductor laser device
according to a sixth embodiment of the invention. In the figure, reference
numerals 16b and 16c designate a lower layer and an upper layer of an n
type InP second current blocking layer including dopant impurities and
impurities serving as hole traps, respectively, and numeral 16a designates
an intermediate layer of an n type InP second current blocking layer
including only dopant impurities. In this semiconductor laser device, the
impurities serving as hole traps substitute for the impurities that are
electrically neutral and interstitial in the crystalline lattice included
in the upper layer and the lower layer of the n type InP second current
blocking layer 16 in the fourth embodiment of the invention. A fabricating
method according to the sixth embodiment of the invention is the same as
in the fourth embodiment, except that InP is grown to include dopant
impurities producing n type conductivity and impurities serving as hole
traps, such as Co, during the epitaxial growth of the upper layer and the
lower layer of the second current blocking layer. In this case, the p type
InP first current blocking layer 12 has a higher charge carrier
concentration than the n type InP second cladding layer 3, and the p type
InP third current blocking layer 13 has a higher charge carrier
concentration than the n type InP contact layer 8. Therefore, when the
substrate is heated during the epitaxial growth to a suitable temperature,
dopant impurities producing p type conductivity diffuse from the first and
third current blocking layers 12 and 13 to the second cladding layer 3 and
the contact layer 8, and the conductivity type of a thin portion of the
second cladding layer 3 and the contact layer 8 comprising n type InP in
contact with the current blocking layers is reversed to p type. In
addition, this diffusion may be carried out by heat treatment after the
epitaxial growth.
In the sixth embodiment of the invention, as shown in FIG. 10, due to the
diffusion of the dopant impurities, a p-n junction between the third
current blocking layer 13 and the contact layer 8, i.e., a p-n junction
with the upper portion of the burying layer, is formed at a position 14a
in the contact layer 8 removed from the interface 9a between the third
current blocking layer 13 and the contact layer 8, i.e., the regrowth
interface with the upper portion of the crystalline burying layer.
Likewise, a p-n junction between the first current blocking layer 12 and
the second cladding layer 3, i.e., a p-n junction with the side surface of
the second cladding layer 3, is formed at a position 14b in the second
cladding layer 3 removed from the interface 9b between the first current
blocking layer 12 and the second cladding layer 3, i.e., the regrowth
interface with the side surface of the second cladding layer 3. Therefore,
as described above, in these p-n junctions, the forward voltage is not
reduced under continuous operation, avoiding deterioration of laser
characteristics.
Meanwhile, since the upper layer 16c and the lower layer 16b of the second
current blocking layer in contact with the third and first current
blocking layers 13 and 12 include the impurities serving as hole traps,
the diffusion of holes from the p type InP first and third current
blocking layers 12 and 13 to the n type InP second current blocking layer
16 is suppressed. Although the impurities serving as hole traps tend to
obstruct the activation of the dopant impurities in the second current
blocking layer 16, the intermediate layer 16a of the second current
blocking layer includes the dopant impurities producing n type
conductivity but it does not include the impurities serving as hole traps.
Consequently, the charge carrier concentration is higher than when the
impurities serving as hole traps are introduced into the entirety of the
second current blocking layer 16, as in the fifth embodiment of the
invention, whereby deterioration of the current blocking effect of the
current blocking layers is further suppressed.
In the sixth embodiment of the invention, deterioration of the laser
characteristics under continuous operation is prevented without reducing
the current blocking effect of the current blocking layers.
In addition, although an InP semiconductor laser device is described, the
same fabricating method and the same structure may be adopted in a
semiconductor laser device comprising another material, such as GaAs. The
number of current blocking layers need not be three.
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